Conductive coating on glass



July 25, 1950 M. J. zUNlcK CONDUCTIVE COATING ON GLASS Filed sept. 20, 1947 Patented July 25, 1950 2,516,663 CONDUCTIVE COATING N GLASS Michael J. Zunick, Chicago, Ill.,

eral Electric X-Ray Corporation,

assignor to Gen- Chicago, Ill.,

a corporation of New York Application September 20, 1947, Serial No. 775,256

6 Claims.

The present invention relates in general to electron ow devices and has more particular reference to iiow devices including X-ray generators, especially those adapted for high voltage operation.

Electron flow devices of the character menv tioned may comprise an electron emitting cathode and a cooperating anode ailording an electron target adapted to receive impingement of electrons emitted by the cathode, the anode and cathode being enclosed in a sealed envelope, usually comprising glass. In the operation of electron flow devices, electrical potential is applied between the anode and the cathode for the purpose o1' driving electrons emitted by the cathode and causing them to impinge upon the anode target. Some of these electrons may deviate from the desired path, between the anode and cathode, and impinge upon the envelope walls. The glass portions of the envelope may thus be subjected to electrostatic charges which may build up upon the envelope suiiciently to rupture the same and thus end the useful life of the device.

An important object of the present invention is to provide means for protecting the glass envelopes of electron now devices against the deleterious effects of electrical stresses to which the same are or may be exposed during the operation of the devices of which they are a part.

Another important object resides in providing the interior surfaces of electron now device envelopes with coatings of desired conductivity to allow the draining olf of electrical charges applied thereto as a result of the operation of the ilow device; a further object being to utilize cor l ductive coatings adapted to resist electron bombardment and also the high temperature to which the same are exposed in the flow device during its operation,

Another important object is to provide the conductive coating at and adjacent the inner wall surfaces of the envelope which are exposed to maximum electrical stress during the operation of the flow device, the coating having limited conductivity whereby to drain oil' the accumulated charges at a rate suilicient to prevent the development of destructive potentials on the envelope;

a further object being to apply the protective coating on the wall surfaces of the iiow device at and adjacent the anode.

Another important object is to provide a conductive, transparent, adherent and non-soluble coating which is stable in vacuum and at high temperatures of the order of 450 C.; a further object being to provide the coating by spraying (Cl. Z-146) the surface to be treated with a solution comprising substantially equal portions by weight of indium chloride and a suitable solvent such as commercial alcohol.

Another important object is to accomplish desired conductivity of the coating by varying the composition of the spray and the temperature at which it is applied, the sprayed on coating being applied to the glass at a temperature of the order of 500 C.

Another important object is to provide an X-ray generator having a glass envelope the inner walls of which are coated with a layer comprising a derivative of an indium salt such as indium chloride forming a conductive coating for draining off electrical potentials occurring as a result of the operation of the X-ray tube of which the envelope is a part.

Another important object is to provide for the successful operation of high tension electron flow devices, in which, necessarily, the distance between the anode and cathode of the device is substantial and in which there is appreciable tendency for wall charges to accumulate upon the envelope walls as a result of electron deviation in traveling the extended path between anode and cathode.

Another important object is to provide an electron ilow device having an elongated sectional envelope of the sort employed in high voltage X-ray generators including a coating of desired limited conductivity, particularly on the inner walls of the envelope sections at and adjacent the anode end of the device, whereby vto protect such sections against destructive voltage stresses 4accumulating thereon through impngement of stray electrons.

The foregoing and numerous other important objects, advantages, and inherent functions of the invention will become apparent as the same is more fully understood from the following description, which, taken in connection with the accompanying drawing, discloses a preferred embodiment of the invention.

VYReferring to the drawing:

Fig. 1 is a view taken longitudinally of a high voltage X-ray generator embodying the present invention;

Fig. 2 is an enlarged view of a portion of the device shown in Fig. 1 and Fig. 3 is a sectional view along the line 3 3 in Fig. 2.

To illustrate the invention the drawing shows an X-ray generator Il comprising an anode I2 and a cathode I3 in opposed relation enclosed taken substantially within an envelope I4 of generally tubular conflgurationthe cathode being mounted at one end and the anode at the other end of the envelope.

The generator is adapted for operation at relatively high voltage values applied between the anode and cathode which, it will be noted, are relatively widely spaced apart at the opposite ends of the elongated envelope struct-ure. The necessary length of the envelope makes difficult the provision of an all glass structure, and consequently the envelope preferably comprises a series of sleeve-like intermediate glass sections I5 and glass end sections I6 and I1 arranged in end to end alignment, the abutting ends of the adjacent sections being interconnected by preferably metallic connector sleeves I8, said connector sleeves having opposite ends forming glass-tometal seals with the facing ends of the glass envelope sections I5, I6 and I'I. At the cathode end of the tube, the end section I'l may be formed with a re-entrant portion I'I' making a glass-t0- metal seal as with a skirt portion of the cathode I3. At theanode end of the envelope, the end section I6 may form a glass-to-metal seal with an annular ring I6 which, in turn, may be sealed to a collar I2 sealed on and forming a portion of the anode element I2. The anode I2, as shown, comprises a tube having a closed end forming a target and an open end exposcd within the envelope and facing toward the cathode I3. The collar I2' also may carry an exhaust iitting I9 adapted for connection as with a vacuum pump for the evacuation of the interior of the envelope, the exhaust fitting being adapted to be sealed ofi and disconnected from the exhaust pump after the generator has been sufliciently evacuated during the making of the same.

It should be understood, of course, that X-ray generators function to develop X-rays at the anode by impingement thereon of electrons emitted at the cathode, such emitted electrons traveling from the cathods in an electron stream directed toward and impinging upon a target 20,-

forming a portion of the anode.

The emitted electrons may be generated and liberated at the cathode as by electrically heating an electron emitting filament, comprising a part of the cathode structure, the electron stream being impelled toward the anode under the driving influence of electrical potential applied, from a suitable 'power source, between the anode and cathode.

In passing from the cathode toward the anode,

electrons may deviate from the desired path and may impinge upon the inner walls of the envelope Il, such impingement of deviating electrons tending to be a maximum at and adjacent the anode end of the envelope. Such electronic impingement upon the envelope walls results in the building up of electrical charges upon the inner surface of the envelope, which, should they become suiliciently large, may puncture the envelope, particularly in the envelope portions adjacent the anode. Electrical charges thus built up on the envelope may also result in the etching of the inner surfaces of the envelope, with consequent weakening thereof. Such charges also, if allowed to remain upon the envelope, may exert an electrostatic eect upon and disturb the iiow of the electron stream to the anode, and such disturbance, in turn, may result in causing further unwanted deviation of electrons and the impingement thereof on the envelope walls.

' Electron deviation may be minimized, to some 4 lenses or screens comprising metallic tubes or sleeve members 2l arranged in spaced relationship within the envelope, between the anode and cathode, and in axial alignment therewith. These lenses are preferably disposed each opposite an envelope sealing ring IB, and supported thereon in any suitable or preferred fashion. The lenses, accordingly, are electrically connected with the connector sleeves I8. and if desired may be electrically biased with respect to the anode or cathode in order to aid in controlling and maintaining electrons in the desired path between cathode and anode.

In spite of the provision of electron lens elements 2l, many electrons may escape from the desired electron flow path and may impinge upon the inner wall surface of the envelope sections I5, I6 and I1, particularly the envelope sections toward the anode end of the envelope, with the undesirable results heretofore mentioned. Where lens elements are omitted the aforesaid undesirable results are, of course, present in full force.

The unwanted results of ambient electron impingement upon the inner walls of the envelope are prevented, in accordance with the teachings of the present invention, by applying to the inner surface of the glass portions of the envelope a transparent, conductive, adherent, non-soluble coating 22. While the coating 22 may be applied to the inner surfaces of all of the glass envelope portions, adequate protection against the destructive eiects of ambient electron impingement on the envelope walls will usually be afforded if the coating is applied only in the sections at and adjacent the anode end of the device. The coating 22 may be applied to the envelope surfaces requiring treatment by spraying the same with a solution comprising an indium salt such as indium chloride and a suitable solvent such as commercial alcohol. The solution may comprise equal parts by weight of the indium chloride constituent and the solvent carrier, and is preferably applied to the glass surfaces requiring treatment while maintaining the glass at a temperature of the order of 500 C. By varying the composition of the spray and the temperature of the glass during the spraying operation, variations in the conductivity of the resultant coating may be obtained, accurate control of the resulting coating, in this manner, being obtainable through a relatively wide resistance range of the order of from 25 to 500 ohms per square.

The exact composition of the resulting coating is not known. It is an end product resulting from the application of the coating solution applied to glass in the manner described. It is surmised that the indium compound decomposes at the temperature of application, and the indium constituent is probably integrally bonded with the glass possibly as an oxide or other compound thereof; and it is thought that the indium decomposition product is integrated with the glass probably by being at least partially dissolved therein at the coated surfaces thereof.

The so applied conductive coating, of cours will be in electrical communication with the metallic connector sleeves I 8 and also with the metal sealing ring I6', whence accumulated charges may be drained oil! from the interior of the envelope walls substantially as quickly as they accumulate. As a consequence, the deleterious results of accumulated envelope wall charges are leliminated. extent, by the provision of a series of electronic u Coatings of the character herein described do not affect the appearance of the device, since the same are transparent. The coatings are substantially inert to heat at the normal operating temperature of the generator, are strongly adherent, are non-soluble, and are substantially unailected by electron bombardment; and the coatings are stable under vacuum conditions.

It is thought that the invention and its numerous attendant advantages will be fully understood from the foregoing description, and it is obvious that numerous changes may be made in the form, construction and arrangement of the several parts without departing from the spirit or scope of the invention, or sacrificing any of its attendant advantages, the form herein disclosed being a preferred embodiment for the purpose of illustrating the invention.

The invention is hereby claimed as follows:

1. An X-ray generator for operation at high voltage comprising an envelope, an anode and cathode mounted in spaced apart relationship on and within the envelope, said envelope comprising glass, and a conductive coating comprising indium adhered upon the inner surface 0f said envelope to drain ofi electrostatic charges that may accumulate on the envelope during and as a result of the operation of said generator, said coating being transparent, substantially inert to temperatures 0i the order of 500 C., and having resistivity of the order of from 25 to 500 ohms per square.

2. An X-ray generator for operation at high voltage comprising a plurality of glass envelope sections and metallic connector rings sealed together in end-to-end relation to form an elongated tubular envelope, an anode and a cathode mounted in spaced apart relationship in and at the opposite ends of said envelope, and a conductive coating comprising indium adhered to the inner surface of said sections at and adjacent the anode end of the envelope.

3. An X-ray generator for operation at high voltage comprising a plurality of sleeve sections of glass and metal sealed together in end-to-end relation to form an elongated envelope, an anode and a cathode mounted in spaced apart relation on and within the envelope, and a conductive coating comprising indium adhered upon the inner surface ofthe envelope and extending in contact with at least a glass and an adjacent metal sleeve section to drain off electrostatic charges that may accumulate on said envelope during operation of said generator.

4. An electron flow device having an envelope comprising glass, an anode and a cathode enclosed within and mounted on said envelope, and a conductive layer comprising indium coated on the inner surfaces of said envelope to depth such that said layer has resistivity of the order of from 25 to 500 ohms per square, whereby to drain oil, at a desired rate determined by the resistivity of said layer, electrostatic charges that may accumulate on said envelope during the operation of the device.

5. A device of the character described having an envelope comprising glass, an anode and a cathode mounted in spaced apart relationship within the envelope, and a transparent conductive coating comprising indium sprayed upon and adhering to the inner surfaces of said envelope.

6. An electron flow device having a glass envelope, an anode and a cathode mounted in spaced apart relationship within the envelope, and a conductive coating comprising indium adhered upon the inner` surface of said envelope for draining electrical charges from the interior surfaces of said envelope.

MICHAEL J. ZUN'ICK.

REFERENCES CITED The following references are of record in the file of this patent:

A. J. Monack, The Glass Industry, Jan. 1947, p. 25. Indium, by S. J. French, Scientic American." Jan. 1937. DD. 20-2l. 

